A MIMO (multiple-input multiple-output) technology is widely applied to wireless communications systems to expand a system capacity and ensure cell coverage. For example, multiple-antenna-based transmit diversity, open/closed-loop spatial multiplexing, and DM-RS-based multi-stream transmission are used for downlink in an LTE (Long Term Evolution) system.
A main process of the DM-RS-based (demodulation reference signal-based) multi-stream transmission is as follows:
Step 1: A base station sends configuration information to a terminal.
Step 2: The terminal performs channel measurement based on the configuration information sent by the base station, and feeds back CSI obtained through measurement to the base station.
Step 3: The base station performs downlink scheduling based on the CSI fed back by the terminal, and sends a physical downlink shared channel (PDSCH) to the terminal based on a scheduling result using a DM-RS.
Currently, two-dimensional beamforming is used for the DM-RS-based multi-stream transmission. That is, a transmit antenna is placed only horizontally, and can generate a beam only in a horizontal direction, as shown in FIG. 1. To further improve multiple-antenna system performance, a three-dimensional beamforming technology emerges accordingly. In comparison with the current two-dimensional beamforming, a degree of freedom in a vertical direction is added in the three-dimensional beamforming. In this way, multiplexing can be performed for more terminals on a same time-frequency resource. Different terminals are distinguished from each other by using a beam in the vertical direction or the horizontal direction, so as to further increase resource utilization or spectral efficiency.
For both the two-dimensional beamforming and the three-dimensional beamforming, the base station usually needs to configure, for each terminal, a plurality of sets of measurement reference signal resources for which a precoding matrix has been used, so that the terminal obtains a set of optimal measurement reference signal resources by measuring the plurality of sets of reference signal resources. The terminal may select a plurality of sets of measurement reference signal resources, and report all CSI corresponding to the plurality of sets of measurement reference signal resources to the base station, so that the base station can manage and coordinate precoding matrices corresponding to measurement reference signal resources of different terminals.
However, a CSI reporting mode specific to each set of measurement reference signal resources is currently configured by the base station. In this case, because candidate measurement reference signal resources are excessive, required CSI report overheads are excessively high, and this solution is not sufficiently flexible.